That’s a good question! It can be tough to make sure a background image fill the whole background, since a site can be viewed on many different screen sizes. For a 27″ monitor, you’re probably looking at an image that’s 2600px wide by 1440px tall. Unfortunately, that still may not cover every case, and it’s a big image for someone to load, especially on a mobile device.
I think a safer bet may be to use the ‘repeat’ setting for the background, and set an image that can be repeated — that way you can use a smaller image, and know that the whole background will always be covered, regardless of screen size. There’s some steps on how to do that here: